diff --git a/Data/String/ToString.hs b/Data/String/ToString.hs
--- a/Data/String/ToString.hs
+++ b/Data/String/ToString.hs
@@ -14,11 +14,15 @@
     ( ToString
     , toString
     , fromToString
+    , prop_fromToString
     )
     where
 
 import Data.String (IsString, fromString)
 
+-- | Class of string-like types that can be converted to 'String's.
+--
+-- Ensure that instances of this class obey the 'prop_fromToString' law.
 class ToString s where
     -- | Convert a string-like type to a 'String'.
     toString :: s -> String
@@ -30,6 +34,11 @@
     toString = (:[])
 
 -- | General coercion between string-like types.
+--
+-- Note that: @fromToString = 'fromString' . 'toString'@
 fromToString :: (IsString s2, ToString s1) => s1 -> s2
 fromToString = fromString . toString
 
+-- | @prop_fromToString x = 'fromToString' x == x@
+prop_fromToString :: (IsString s, ToString s, Eq s) => s -> Bool
+prop_fromToString x = fromToString x == x
diff --git a/to-string-class.cabal b/to-string-class.cabal
--- a/to-string-class.cabal
+++ b/to-string-class.cabal
@@ -1,5 +1,5 @@
 Name:                   to-string-class
-Version:                0.1
+Version:                0.1.1
 Synopsis:               Converting string-like types to Strings.
 Description:            This library provides the class:
                         .